There has been much interest concerning the role of genes that transform rodent cells in human neoplasia. In our laboratory, human bronchial epithelial cells have been infected by Kirsten sarcoma virus with an amphotropic murine leukemia virus helper. This viral infection results in epithelial cell colonies which do not differentiate in response to serum as do normal epithelial cells. These transformed cells are currently being further characterized. Temperature-sensitive mutants of Kirsten sarcoma virus, and other RNA viruses with transforming genes, are being studied in transformation assays.